Unfold 2 polygons into a plane, without distorting the dimensions of the polygons
This is a fun question, so let me show my limited knowledge :).  

1) Edge angle
You can display the angle of the bend by enabling Edge Angle in the Overlay menu.
It's only visible in Edit Mode, and you have to have the edge selected.



2) Unfolding into 2D
Very fast and very dirty way for the unfolding bit:
If you unwrap your mesh with Smart UV project, you will get it unfolded as a UV map.
If you do it right, it will perfectly correspond to the shape of the faces.
You can then  export it as a .png,.svg or eps. 

Unwrap your mesh with Smart UV project
  

Export your UV map as svg.




Note:
This is definitely a "solution".
But maybe the Blender Elders will banish me to hell for this.